1. Field of the Invention
This invention relates generally to gas separation by electrostatic precipitators and more particularly to baffle assemblies used within particle collection hoppers of electrostatic precipitators for reducing re-entrainment of the particles into the gas stream without interfering with emptying of the hoppers.
2. Description of the Prior Art
The removal of particles from a gas stream by electrostatic precipitators is well known in the art. A typical electrostatic precipitator of the plate type which removes particles from a gas flowing therethrough is illustrated in Ragland U.S. Pat. No. 3,425,190. In such electrostatic precipitators, gas flows through an inlet port into the precipitator and through gas passages formed between rows of vertical collecting plates. Some electrostatic precipitators are extremely large and usually include more than one set of collecting plates throughout the precipitator. Each set contains a plurality of spaced collector electrodes. There may be any number of sets of collector electrodes in the precipitator, depending on the size, shape, and degree of precipitation that is desired. Suspended within each of the gas passages in each set of the collector electrodes is a plurality of discharge wire electrodes which are electrically insulated from the shell. As the gas passes through the gas passages between the collector electrodes, the discharge electrodes ionize the particles in the gas which are then attracted to and collected on the vertical collector electrodes. The particles which collect on the collector electrodes are removed therefrom in any conventional manner such as by rapping the electrodes to cause the particles to be dislodged therefrom and fall to the bottom of the precipitator. As shown in the Ragland Patent, the bottom of the precipitator contains a plurality of hopper bins in which the dust particles fall, are collected, and then removed through the bottom of the hopper to the outside of the precipitator.
Ideally, one hopper should be suspended under each set of collector electrodes in order to collect those particles which are removed from the collector electrodes in each set. In this manner, gas flowing beneath the collector electrodes, and thus not subject to precipitation by the discharge electrodes, would only flow below one set of collector electrodes before being forced back up into the gas passages of the remaining sets of collector electrodes by the walls of each hopper. In this way, maximum precipitation of the entire gas is assured and any re-entrainment of the particles in the gas flowing beneath one set of the collector electrodes will be acted upon by the discharge electrodes in the remaining sets of collector electrodes.
Although it is preferred to have only a single hopper below each set of collector electrodes, it has been found that it is impractical to do so because of the high cost of manufacturing and installing an individual hopper under each set of collector electrodes and because of the cost of the machinery and apparatus which is needed for hook up to the bottom of the precipitator to remove the particles for disposal outside the precipitator system. Therefore, alternate means of removing the particles from the electrostatic precipitator has been devised, the most widely used alternate being to have one hopper bin placed under two sets of collector electrodes. This decreases the number of hoppers used in an electrostatic precipitator by one-half. However, the gas stream tending to flow beneath the collector electrodes within each hopper bin by-passes two sets of collector electrodes before it is forced back up into the gas passages. Since the gas stream by-passes two sets of collector electrodes before being forced back up into the gas passages, particle collection is not as efficient as it would be if the gas stream were allowed to flow under only one set of collecting electrodes before being forced back into the gas passages. Therefore, common practice now is to suspend rigid baffles within each hopper between adjacent sets of collector electrodes which extend to a point just above the bottom of the hopper bin. This baffle system obstructs the flow of gas beneath the collector electrodes and forces the gas back up into the gas passages after it flows beneath only one set of collector electrodes.
However, a baffle assembly in each hopper bin between adjacent sets of collector electrodes does have some distinct disadvantages. One disadvantage is that the opening of each hopper bin at the outlet end is extremely small and a rigid baffle extending down to the outlet end effectively cuts in half the space through which the particles must pass for disposal. The particles which fall into the hopper on each side of the baffle assembly tend, in many circumstances, to lodge between the sides of the hopper and the baffle and will not fall through the outlet end. In addition, when particles collect and cool in the hopper, they tend to form a sticky and thick agglomerate which tends to form a bridge between the wall of the hopper and the baffle. If the particles are allowed to cool too long, they become extremely hard and prevent the discharge of the particles from the hopper. In such a situation, the solid bridge of particles between the hopper and the baffle assembly must be broken loose manually by jackhammers or any other conventional means. This, of course, requires the precipitator to be shut down which is undesirable. To prevent particles from lodging between the wall of the hopper and the baffle assembly, and to prevent the bridging action of cooling particles, the baffle assembly may extend to above the end of the hopper. However, it has been found that once the particles have been emptied from the hopper, gas flowing beneath the collector electrodes will also flow down and around the lower end of the baffle assembly and back up the other side; thus, the gas will not be forced back up into the gas passages until it has passed beneath two sets of collector electrodes. Gas flowing around the end of the baffle assembly defeats the purpose of the baffle system.